Wireless communications systems, including cellular phones, paging devices, personal communication services (PCS) systems, and wireless data networks, have become ubiquitous in society. To attract new customers, wireless service providers continually seek to improve wireless services cheaper and better, such as by implementing new technologies that reduce infrastructure costs and operating costs, increase handset battery lifetime, and improve quality of service (e.g., signal reception).
CDMA technology is used in wireless computer networks, paging (or wireless messaging) systems, and cellular phones. In a CDMA system, mobile stations (e.g., pagers, cell phones, laptop PCs with wireless modems) and base stations transmit and receive data in assigned channels that correspond to specific unique codes. For example, a mobile station may receive forward channel data signals from a base station that are convolutionally coded, formatted, interleaved, spread with a Walsh code and a long pseudo-noise (PN) sequence. In another example, a base station may receive reverse channel data signals from the mobile station that are convolutionally encoded, block interleaved, and spread prior to transmission by the mobile station. The data symbols following the interleaving block may be separated into an in-phase (I) data stream and a quadrature (Q) data stream for QPSK modulation of an RF carrier. One such implementation is found in the 1xEV-DV version of the IS-2000 standard. Similar implementations are found in IS-95 standard wireless networks.
In order to improve reception at the mobile station, wireless networks frequently employ transmit diversity, which uses two antennas—a main antenna and a diversity antenna—to transmit data in a forward channel (or downlink) to a mobile station. Even if incoming signals from one of the antennas are in deep fade, the spatial separation of the two base station antennas ensures that the mobile station receives signals from the other base station antenna that are not in deep fade.
Different forms of transmit diversity exist. Examples of transmit diversity include: 1) orthogonal transmit diversity (OTD); 2) space-time block codes (STBC) (e.g., Alamouti codes); and 3) phase sweep transmit diversity (PSTD). Typically, these prior art diversity schemes require either modification of the mobile station or continuous transmission of the overhead channels from both antennas, thus doubling the overhead. Moreover, a base station typically requires two RF transmit paths in order to process signals from the main antenna and the diversity antenna. However, this is very expensive solution.
Therefore, there is a need in the art for improved apparatuses and methods for using transmit diversity in a wireless network. In particular, there is a need for a CDMA base station that uses transmit diversity to communicate with mobile stations without requiring extensive modification of either the mobile stations or the base station.